powerful notebook computers and PDAs have created a new mobile computing environment. Because of its already well established market acceptance, IEEE ...
Enhancement of IEEE 802.11 Distributed Coordination Function with Exponential Increase Exponential Decrease Backoff Algorithm Nah-Oak Song, Byung-Jae Kwak, Jabin Song, Leonard E. Miller Advanced Network Technologies Division National Institute of Standards and Technology, MD, USA Emial: nsong, bjkwak, jabin, lmiller @antd.nist.gov
Abstract— A new backoff algorithm is proposed to enhance the performance of the IEEE 802.11 Distributed Coordination Function (DCF) which employs Binary Exponential Backoff (BEB) algorithm. The proposed algorithm, called the Exponential Increase Exponential Decrease (EIED) backoff algorithm, is quite simple to implement while significantly improving the network performance over BEB. Another backoff algorithm called Multiple Increase Linear Decrease (MILD) backoff algorithm is considered for performance comparison. The simulation results show that EIED outperforms BEB and MILD in terms of both throughput and delay. The performance gain of EIED comes from successfully balancing the two extreme backoff policies of BEB and MILD. Index Terms— Wireless LAN, DCF, Binary Exponential Backoff, EIED, MILD.
I. I NTRODUCTION Combined with wireless communication technologies such as Wireless LAN (IEEE 802.11), Bluetooth, and HIPERLAN, powerful notebook computers and PDAs have created a new mobile computing environment. Because of its already well established market acceptance, IEEE 802.11 is the most successful among the above mentioned wireless communication standards. The Distributed Coordination Function (DCF) is the fundamental access mechanism in IEEE 802.11 Medium Access Control (MAC) while the Point Coordination Function (PCF) is used optionally [1]. In DCF, Binary Exponential Backoff (BEB)1 is employed as a stability strategy to share the medium. At the first transmission attempt of a packet, BEB selects a random slot from the next CW CW slots with equal probability for transmission, where CW is the minimum contention window size. Every time a node’s packet is involved in a collision, the contention window size for that node is doubled up to its maximum CW , that is, CW
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and the new contention window is used for the following transmission attempt. A node resets its contention window 1 In IEEE 802.11, a truncated BEB is used. Many papers refer to this truncated version as BEB. We follow the convention in this paper.
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Backoff mechanism of BEB: CW
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to the minimum after a successful transmission, or when the total number of transmission attempts for a packet reaches ; for basic access mechanism and the limit : (: := for the Request-To-Send/Clear-To-Send (RTS/CTS) exchange mechanism). However, the contention window resetting mechanism causes a very large variation of the contention window size, and degrades the performance of a network when it is heavily loaded since each new packet starts with the minimum contention window, which can be too small for the heavy network load. Fig. 1 illustrates the backoff mechanism of BEB, where CW ?A@CB and CW DE@CFGH= (:
